Catalytic reactors employing sensitive catalyst materials are found in various processes. Regeneration of industrial catalysts often require severe operating conditions, especially where the catalyst is employed in conversion of organic feedstocks and process coke or the like is removed by high temperature treatment and/or oxidative regeneration. Recent efforts have been made in the field of gasoline blending to increase gasoline octane performance without the addition of deleterious components such as tetraethyl lead and benzene. It has been found that lower molecular weight unsymmetrical ethers such as MTBE and TAME can be added to C.sub.5 -C.sub.10 hydrocarbon-containing gasoline products in order to increase octane number. The research octane number (RON) of MTBE has been listed at 115 (Lander, E. P. et al, "National Petroleum Refiners Association Annual Meeting", San Francisco, Calif., Mar. 20-24, 1983). The blending octane number of MTBE has been calculated over various concentrations and some of the readings are: RON, 115-135; MON (motor octane number), 98-110; and (RON & MON)/2, 106-122.5 (Pecci, G. et al, Hydrocarbon Processing, 1977, 56, 98). Blending octane number rises when MTBE concentration is decreased and saturates concentration cf the base fuel is increased.
Conventional etherification processing uses as catalyst a macroreticular cation exchange resin in the hydrogen form. An example of such a catalyst is "Amberlyst 15". A resin catalyst gives a high conversion rate but is unstable at elevated temperatures (above about 90.degree. C.). When overheated, the resin catalyst releases sulfonic and sulfuric acids. In addition leaching of acid substances from the resin catalyst even at normal operating temperatures causes a reverse reaction--decomposition of ether products to starting materials--to occur upon distillation of ether product. Overall yield is thereby significantly decreased (see U.S. Pat. No. 4,182,913 to Takesono et al).
Etherification reactions conducted over a resin catalyst such as "Amberlyst 15" are usually conducted in the liquid phase below a temperature of about 90.degree. C. and at a pressure of about 200 psig. Equilibrium is more favorable at lower temperatures but the reaction rate decreases significantly. Also excess methanol appears to be required to achieve acceptable selectivity over "Amberlyst 15" (see Chu et al, Industrial Engineering and Chemical Research, Vol. 26, No. 2, 1987, 365-369).
Some recent efforts in the field of etherification reactions have focused on the use of acid medium-pore zeolite catalyst for highly selective conversion of iso-olefin and alcohol starting materials. Examples of such zeolite catalysts are ZSM-4, ZSM-5. ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-50 and zeolite Beta. Due to lower acidity as compared to resin catalysts, the zeolites need to be employed at higher reaction temperature to achieve the desired conversion rates. These solid acid catalyst particles are much more thermally stable than resin catalyst, are less sensitive to methanol-to-isobutene ratio, give no acid effluent, and are easily and quickly regenerated (see Chu et al, "Preparation of Methyl tert-Butyl Ether (MTBE) over Zeolite Catalysts", Industrial Engineering and Chemical Research, op cit.).
Certain processes, such as MTBE production, can employ catalyst packed in a distillation column to achieve reaction and distillation concurrently in the same unit operation. This technique is disclosed in U.S. Pat. Nos. 4,215,011 and 4,232,177 (Smith) and 4,935,577 (Huss et al). In such configurations, in situ regeneration or reactivation of catalyst may not be feasible.
It is an object of the present invention to provide apparatus for continuous operation in preparation of t-alkyl ethers from an alcohol and an iso-olefin with a conventional acid resin catalyst whereby the resin catalyst is protected from impurities such as nitrogen compounds, metals, and coke. It is a further object to provide a multistage catalytic reactor system employing regenerable catalyst in a primary reaction zone and having non-regnerated catalyst in a secondary reaction zone.